1. Field
This disclosure relates generally to photovoltaic (PV) cells and, more specifically, to multi-junction PV cells.
2. Description of the Related Art
PV cells absorb energy from electromagnetic waves and convert the absorbed photon energy into electrical energy. An important use of PV cells is converting solar energy into electricity. There are two broad classes of structures used in PV cells: single-junction and multi-junction cells.
A single-junction PV cell comprises a single subcell having a characteristic band gap. The subcell typically comprises one or more semiconductor materials having two oppositely doped regions, a base region and an emitter region, contacting at the junction. The subcell is exposed to a source of electromagnetic radiation. When the energy of an incident photon is within or greater than the subcell's characteristic band gap, the photon can be absorbed by the subcell to create an electron-hole pair. The electrons and holes are then driven by the internal electric field in the subcell to produce a current.
Single-junction PV cell efficiency is limited due to the single subcell's inability to efficiently convert the broad range of energy within the electromagnetic spectrum. Photons with energy below the subcell's band gap are lost, because they pass through the subcell without being absorbed. Photons with energy above the subcell's band gap are also lost. The energy necessary to generate the hole-electron pair is absorbed, but the remaining energy is converted into heat.
Multi-junction PV cells are a type of PV cell developed for improved efficiency over single-junction PV cells. By incorporating different semiconductor cell materials, the band gap of each subcell in a multi-junction PV cell can be selected to absorb a specific range of photon energies. Multi-junction PV cells are so-called because the cells comprise multiple subcells, each subcell having at least one junction between a p-type doped region and an n-type doped region.
The subcells in a multi-junction PV cell are typically stacked so that electromagnetic radiation falls first on the subcell having the highest band gap. Photons not absorbed by the first subcell are transmitted to the second subcell, which then absorbs the higher-energy portion of the remaining photons while remaining transparent to the lower-energy photons. Photons below the band gap of the second subcell pass through to the lower subcells to be absorbed there. These selective absorption processes continue through to the final subcell, which has the smallest band gap.
In commercialized multi-junction PV cells, the subcells are electrically connected in series through interconnect layers, and the composite PV cell has two terminals, a front contact and a back contact. Because of the series connection, the subcell with the lowest current limits the overall current of the cell. Consequently, if the maximum current of each subcell is not the same, then cell efficiency suffers. To improve cell efficiency, cell designers adjust the subcells' thicknesses to ensure each subcell is generating the same current.
It is difficult to have each subcell operate at the same current, and thus maximum efficiency, if the multi-junction PV cell is to be used in variable lighting conditions. For example, a cell designer can adjust subcell thickness to ensure each subcell generates the same current in full, direct sunlight. However, the electromagnetic spectrum for full, direct sunlight is different than the electromagnetic spectrum for partial sunlight and for indirect sunlight. If the PV cell is operating on a cloudy day, the number of low-energy photons in the electromagnetic spectrum will be reduced, leading to fewer photons being absorbed by one or more of the subcells. Consequently, on a cloudy day, each subcell will not generate the same current. The overall cell current will be lowered due to the current-limiting lower-most subcells.
Thus, a problem remains in maintaining cell efficiency in multi-junction PV cells over variable lighting conditions.